I dont have a clarinet yet but I hope to get one in the future, but I was wondering why beginners instruments are in Bb? Why are the flat keys more predominant?

Some time in the 18th or 19th century Bb was selected as the common tuning of instruments used in the military marching bands in Europe. The rest is history and conservative conventions.

Since most music for clarinets are written for Bb instruments that is a good choice for a beginner as well as for professionals.

If you plan on playing in a symphony orchestra you will probably be required to have a Bb and and A clarinet, sometimes changing in the same piece. There are other tunings, as example C, but they are quite unusual. Other sized of clarinets, as example alto or bass clarinets are in other keys.
/Gunnar

I am sorry, I am not a clarinet player but why Bb?! Notes are called C, D, E, F, G, A and H! B is Hb. Bb equials "Hbb" and this equials A...

B or H. Bb or B.
This is simply two different conventions.
Music has a long history and has moved in different directions in different countries.
The German note names, used to be followed in my home country Sweden: AHCDEFG. H becomes B when lowered a half step.
The English note names, followed in USA as well: ABCDEFG. B becomes Bb when lowered a half step.
The French note names, similar in most latin speaking countries: la, si, do or ut, re, mi, fa, sol. Si lowered a half step becomes si bémol.

/Gunnar

H is German for B

Oh, I am sorry!Thank you very much for explanation! Just got confused...

No need to apologise - many people get confused. The first time they see 'clarinet in B' (or trumpet or horn) they panic & expect to have to transpose the part.

Aside from Gunnar's excellent responses, everything you encounter at the beginning is going to be for Bb clarinet if you are in a traditional western music setting. You will be rather advanced (within the amateur realm) before you starting hitting music for Eb or A clarinet. This is the most practical reason you would want to stick with convention and purchase a Bb clarinet and why they are what are mostly sold at brick-n-mortar music stores.

Well I finally got a clarinet, a Windsor Bb. I can't play it yet tho as Im full of cold with a chesty cough that I need to get rid of first.

Congrats! Here's to a speedy recovery and good reeds!

Thanks